This invention relates to Raman spectroscopy in general and Raman spectroscopy in the presence of fluorescence in particular.
It is known that certain materials exposed to light radiation will emit both radiation due to Raman scattering and fluorescence.
Raman scattering is a result of quantum exchange of energy between the photons of the primary radiation and the molecular system. Information such as material identification may be obtained by spectrum analysis of the Raman scattering.
Fluorescence is the property of some materials to emit photons due to the excitation and subsequent remission by electron energy levels. If the material is fluorescent, it is possible that a background of fluorescence occurs at the same frequency as the Raman scattering, degrading the sensitivity of Raman spectroscopy. It is, therefore, highly desirable to suppress or discriminate against the effects of fluorescence. At least one effort in the prior art has been successful in obtaining this goal.
U.S. Pat. No. 3,807,862 Hatzenbuhler discloses apparatus and method for Raman spectroscopy in the presence of fluorescence. Both the Hatzenbuhler device and the present invention are based upon the phenomenon that under certain circumstances Raman scattering will be substantially polarized, but the fluorescence is comparatively random or non-polarized. The particular circumstances are that: the primary radiation source be plane polarized, as would be a conventional laser; the material sample be freely and rapidly rotating as would be small molecules suspended in a liquid or gas; and that the time constants be such that the Raman scattering takes place faster than the rotation of the molecules and the fluorescence slower than the rotation of the molecules.
The Hatzenbuhler apparatus uses a single non-polarized detector, and a plane polarized laser as the primary radiation source. A polarization rotator is interposed between the laser and the material sample and switched so as to provide alternate planes of polarization. Hatzenbuhler discriminates against fluorescence by separately detecting all the light propagating with polarization parallel to the laser polarization and then perpendicular to the laser polarization. If the intensity of the non-polarized fluorescence is the same in both directions, then the difference between the two signals obtained is a measure of the strongly polarized radiation due to Raman scattering.
The present invention provides an improved apparatus over that of Hatzenbuhler. A particular advantage of applicants invention is its compatibility with spectrum analysis systems such as disclosed in U.S. Pat. No. 3,914,055 by Wolga et al. When the teachings of the present invention are followed, the result is fluorescence suppression with a minimum of modifications or additions.